If you've ever stopped to peer with suspicion at the sky, wondering how that would go, a mammoth piece of space rock on its way in to bring ruin, the last two years have not been especially restful. In February 2013, a large asteroid ripped over the Chelyabinsk district of Russia, trailing cartoonish lines of smoke as it made its shallow entry, radiating so much light and heat that onlookers were left with reddened faces. Skin peel. When the asteroid exploded, 15 miles up, there was a terrible, prolonged bang – a noise that has rung on, in its way, ever since.

We now know that the explosion over Chelyabinsk occurred with a force equal to 500 kilotons of TNT, or a couple of dozen Nagasaki bombs. Had it come down a little steeper that February, directing the might of its detonation at rather than over Chelyabinsk, the asteroid would have killed thousands on the ground. A little later, it might have done for many more in Moscow, or Riga, or Gothenburg. Though nobody died at Chelyabinsk, it was an event of such calamitous potential that the asteroid was classified by certain astronomers a "city-killer". Those astronomers have wondered, since, if we're not being a little complacent.

To recap: asteroids are hunks of space rock that whisk around the solar system in orbits around the sun, colliding with anything that crosses their path. If they collide with Earth, we call them meteorites. Most are small and burn up in our atmosphere; some are big enough to matter, such as the Chelyabinsk rock, which was the size of a swimming pool, 20m from end to end. Though Nasa has for some time been tracking giant asteroids (those at least 1km wide), it has never seemed much concerned about lesser rocks – those capable only of scraping away a city, say.

At a press conference earlier this year, former Nasa astronaut Dr Edward Lu announced that there are around 1m asteroids in the Earth's vicinity "with the potential to destroy a major metropolitan area". He teed up an animated graphic to demonstrate how unprepared we are. The graphic showed the Earth in orbit among the dangerous asteroids we knew about and were tracking, around 10,000 of them. Seen like this, our planet looked like a pedestrian hustling along a busy street, not overly troubled. Then Lu changed the graphic to show "what it really looks like out there" – the Earth ploughing on through a million-strong field of city-killing asteroids. I saw the same pedestrian, now trying to make it across a train station concourse in the middle of rush hour, avoiding collisions purely by fluke. "Blind luck," as Lu put it.

This information, I thought, watching online, was appalling. Why wasn't it all over the nightly news? I can't be the only person who feels fidgety on the subject, having watched Deep Impact and Armageddon at an impressionable age. I watched some of the YouTube videos of the Chelyabinsk strike, dozens of them recorded on mobile phones, and found that though the images were shocking (people swept flat by the shock of the impact), it was the noise that was truly unbearable. The meteorite's thoom rang on for longer than made sense; it sounded unnatural, or maybe too natural. It seemed to contain an old message. Don't get comfy, Beijing. Look alive, London.

"Feel the heat coming off that," a meteorite expert called Peter Jenniskens said to me. We stood together watching a tower burn, fire spouting out of it as a troupe of firefighters prepared to approach and subdue the flames. It was a warm day, doubly sweaty in front of the tower. "People in Chelyabinsk felt heat like this," he said, "looking up."

It was April, days after Lu's press conference, and I was at a firefighters' training compound in Texas, north of Houston. A documentary called Disaster Playground was being filmed on the site; led by experimental film-maker Nelly Ben Hayoun, its crew had travelled the world asking catastrophe specialists to simulate on camera how they'd behave if an asteroid were to come in and raze their home town. The day I joined the crew, they'd set up in Texas to focus on the likely reaction of the fire crews and response teams who'd be first on the scene to face… what, exactly?

If I wanted to know what a city-killer would look like, here it was. The training compound was just like a theme park, only one devoted squarely to unhappy things, an expanse of Texan scrub transformed into a town that had endured devastation on multiple fronts. Piles of rubble represented collapsed houses. Warehouses belched black smoke. There were plastic corpses strewn about, as well as live actors wearing fake blood. Crushed vehicles were parked next to pieces of aeroplanes and there was a full-length train, derailed, its buffet car at an angle atop its sleeper.

I watched from behind the cameras as a group of specialists, firemen, operation chiefs, a communications officer and Jenniskens spent a day simulating death from above. An operations chief wondered where best to park a command vehicle – up wind or down wind from smoking space rock? – until Jenniskens pointed out, kindly, that there wasn't likely to be much space rock left unevaporated. Everyone anticipated bodies. If a city-killing asteroid made land, or burst apart low enough in the sky, "it would be similar to what you'd have after a nuclear explosion", Jenniskens said. "You're standing on a planet that is moving at 30km a second around the sun. If there's anything in our way, it's hitting us at 30km a second. That energy has to go somewhere."

An astronomer employed by the Seti (Search for Extraterrestrial Intelligence) institute in California, Jenniskens had flown to Texas especially for the shoot. He had thinning hair under his baseball cap, and wore round spectacles and a polo shirt with irretrievably crumpled collar wings. Born and raised in Holland, his English is pleasantly accented, a gentle singsong. "I hope I never see this in real life," he told me as we toured the disaster site together. The 52-year-old has spent most of his adult life watching the skies, counting, charting, anticipating. In the course of his studies, he's "looked at around 175,000 meteorites. Twice."

He especially likes to hunt meteorites on the ground, and in 2008, after an asteroid exploded over Sudan, he led the team that searched the Nubian desert for fragments. There was some warning before the Sudanese rock arrived, an astronomer having spotted it around 20 hours beforehand; Jenniskens booked a flight. When he travelled to Russia last year, to try to recover pieces of the Chelyabinsk asteroid, he was necessarily slower off the mark. The Russian strike happened without warning: coming in from the direction of the sun, it was invisible even to telescopes pointed in the right direction. The entire space community had to give a collective shrug and admit they found out about it on Twitter.

Jenniskens has the scientists' habit of crisp and demoralising literalism, describing the death of the dinosaurs as only Earth's "most recent" asteroid-brought extinction. For a jumpy civilian like me, conversation with Jenniskens was fraught; but he knows as much about the subject as anyone alive, and if there was cause to be nervous he would know. Do meteorites scare him? "I'm a scientist!" Jenniskens said, meaning no. They thrill him. Anyway, he said, the rocks with the most lethal potential collided with Earth every 10m or 100m years. None was on course to hit us for 200 years at least. "The really big impacts are infrequent," he assured me. Together we inspected a burning chemical tanker. "It's the pesky ones you could start to worry about."

Chelyabinsk in 2013. Sudan in 2008. Impacts over Indonesia, Egypt, Australia, Argentina, California and Finland; four in the north Pacific and three in the south, two in the south Atlantic and one in the north, four in the Indian Ocean, one in the Med, one in the Arabian Sea, one in the Tasman and one off Antarctica. Call them what you like – "city‑killers" or "pesky ones" – there have been 26 meteorite strikes since the turn of the century that were large enough to cause a kiloton-class pop.

Lu believes we should be worried by that. "Every single day," he told me, "we roll a dice. And on most days? Nothing large hits the Earth."

Lu and I met over Skype, not long after he'd given his press conference in Seattle. A stocky, 51-year-old Asian American, with military-short hair and a bulbous nose, he spoke from his San Jose home, having just dropped his kids off at swimming class. He wore a yellow shirt with a spaceship decal stitched on to the breast, a keepsake from his days as a Nasa astronaut. Behind him I could see pieces of meteorite arrayed on shelves, next to a toy model of that other city scourge, Godzilla.

I told him how nervous his press conference had made me. Lu chuckled and said, "I don't like to dwell on the worst things that could happen." Then he briskly worked through a scenario that had the Chelyabinsk asteroid explode over London, causing a collapse of the world economy, slinging humankind back to the dark ages. Though he'd rather inspire people than scare them, Lu insisted, he felt obliged to talk this way. "How else do you wake people up so we take a problem seriously before a million people are killed?"

Inside the response room. Photograph: Disaster Playground

Only one person on record has ever been struck by a meteorite. Mrs Elizabeth Hodges, a housewife in Alabama, was bruised on the hip by a bowling ball-sized rock that fell through her roof in 1954. A teenager in Uganda was meant to have been pinged in the head by a bullet-sized meteorite two decades ago, but he has never been properly identified. There have been other claims of encounters with space rock: a Suffolk grandmother nicked on the arm in 2004, a German kid burned on the hand in 2009, a Norwegian skydiver almost scythed in half in 2012, all ultimately downgraded to cases of wishful thinking. If meteorites were a threat, I said to Lu, why had nobody been killed by one?

"Human civilisation is not that old," he replied. "Only in the last couple of hundred years has the population skyrocketed." We had placed ourselves in unprecedented danger, he suggested, by spreading ourselves so thickly around the world. "The effects of an impact are going to be much, much worse than they would have been 1,000 or 2,000 years ago. Drop a large asteroid in the middle of the Pacific back then? You lost a bunch of fishing villages. It was a story about a flood. Do it today? You lose Tokyo. Los Angeles. Sydney."

When I got in touch with Lu's former employers to ask if we should be worried about meteorites, a Nasa spokesman hedged. "Not yet," said Don Yeomans, a senior figure in the Near-Earth Object Programme, "but we're a long way from finding and tracking them all." As the world's most prominent space agency, Nasa has had to walk a fuzzy line on asteroids. On the one hand, it can't be seen to be doing nothing about an obvious, provable threat from the solar system. On the other, the matter carries with it a whiff of hobbyhorsing, of paranoia. Nasa commits a tiny proportion of its budget to "planetary defence", a fraction of 1%; there are sexier projects out there, like getting feet on Mars. Even so, gestures have been made: a few years back, it set up a Twitter account, @AsteroidWatch, to deliver updates on near-misses as well as tidy pleasantries about our general safety.

In April, an anxious tweeter was told by AsteroidWatch that Nasa was "watching the Earth's back". Lu was sceptical. "I understand there's a natural tendency to believe, 'Oh, there must be smart people working on this.' The truth of the matter is, the governments of the world, the government space agencies of the world, are currently not addressing it." Lu once helped invent something called a "gravity tractor" – a craft that could be launched into space at an asteroid due to collide with us, subtly to tug at its orbit so that it would miss the Earth. Such an effort would have to be undertaken years if not decades in advance in order to be effective, and though Lu's gravity tractor is generally agreed to be our most plausible shot at diverting an incoming threat, we'd never have time to launch it as things stand.

There was less than a day's notice before the Sudan asteroid hit back in 2008, but at least that was some warning. The rock was spotted by an amateur skywatcher from an observatory in Tucson, the first time in the Earth's history that inbound rock had been identified before it struck. (The second time in history was last New Year's Eve, when that car-sized lump came into view on its way towards the Atlantic. It was seen by the same skywatcher in Tucson, who'd fortunately chosen to sit by his equipment as midnight approached, rather than go to a party.)

Lu doesn't believe this is good enough. He guessed it would cost roughly as much as a new shopping centre to build and maintain the technology to find and track the million city-killers out there. Lu knew the figures because he'd spent years trying to get just such a project off the ground. "You could split up all of human history into two phases," he said. "One where you're subject to this random control-alt-delete of life on Earth. Or the phase that we are just entering now, where we can put a stop to it."

He explained something called the Sentinel mission, a plan to launch, in 2018, a privately funded satellite that would orbit the sun near Venus and send back data about asteroids that were on course to whack us. Millions of dollars had been raised for the Sentinel already, but Lu and his colleagues needed more. The Sentinel mission was nonprofit, Lu was careful to state; though he was paid for his advocacy work, he had taken "a substantial pay cut" to be a part of it. His previous job had been at Google, where Lu helped develop the Google Earth application. He changed careers because "sometimes there are things that are so important that they just need to be done".

I asked if becoming a parent was also a factor. Lu said: "That, and having seen the Earth from space. You realise this is all we've got."

Plastic body parts at the scene of the simulated strike. Photograph: Disaster Playground

I had flown to Texas to watch Disaster Playground being made with the idea that it would increase my appreciation of human competency, our readiness to cope with the threat from space. If I could get a sense of how the ground reaction to a city-killer would play out – ideally, in my mind, the hurried assembly of prime earthly talent, as per Armageddon and Deep Impact – it might offset the lingering memory of Cormac McCarthy's book The Road, which went big on post-apocalyptic ruin and hopelessness.

As the documentary's simulation pushed into the afternoon, however, filming became increasingly stressful. There was a mock press conference in which the communications specialist, a jowly Texan called Will Welch, gave advice on the kind of language people would need to hear in the wake of catastrophe (dry evasion, mostly), as well as the kind of language that might inadvertently send them running for the hills. I watched a fireman called Dornhoeffer, his heroic surname printed in large letters on the tail of his treated coat, put himself into character as a first-responder brought to quivering dread by the horror of it all. "Oh God," said this rhino of a man, his voice cracking, "multiple victims. Multiple victims! Does anybody copy? Bring everything."

Like Dornhoeffer, the disaster specialists assembled for the day kept having to improvise. Few seemed to have given the prospect of a meteorite strike much thought; there was no official procedure to draw on. And I guessed that if this group of genuine proficients, most of whom had recently answered the threat of killer tornadoes, killer spills, killer explosions, had no methods in place to cope with killer asteroids, it was a good bet the responders in other world cities didn't either.

Only Welch seemed to have a plan. When he sat down for his on-camera interview, he was prompted to imagine the very worst kind of asteroid impact, and said one of the first things he'd do was call his wife. The director urged him to enact how this conversation might go, so Welch took out his mobile phone. "Sweetheart," he said, "let's meet at our primary location. Grab the safe box with the documents… Screw the CB radio on to the car… I'll be on our designated channel." The director was surprised. Welch and his wife had a designated CB channel? Yes; also, he kept a locked box in the bed of his truck containing water, self-heating food parcels, sleeping bags and spare pairs of his wife and kids' shoes.

Shit, I thought, how quickly we'd got there – to shoe hoarding. I could picture it suddenly, after the rock came down. Our subsistence in the hands of guys such as this, with their locked boxes of food. These were the things McCarthy had imagined. According to The Road's timeline, it wouldn't take long before the psychopaths assumed control. "Within a year there were fires on the ridges," goes one memorable line, "and deranged chanting."

On the fringes of the Texas training yard, next to a collapsed house where an actor streaked with artificial blood was shouting, "Somebody help me!", I took out my phone. It suddenly seemed very urgent I call my wife, to ask if she thought we should choose a designated CB channel of our own. On a nearby stretch of ground, I saw the uniforms of a dozen firefighters left in piles, jackets laid over boots with the sleeves neatly folded. From where I stood, the piles looked like firefighters who'd given up and were prostrate, covering their heads.

At breakfast the next day, Jenniskens suggested we head out into the Texan wilds. He knew a place where we might find remains from the biggest known asteroid ever to hit the US. Two miles wide, it came down 35m years ago and caused such a mess that there was still debris to be found in some places, including in the bed of an ancient river about 40 miles from our hotel. "It's different," Jenniskens promised, "when you can feel this stuff in your hands."

We hired a taxi and went asteroid hunting. Our driver, an overweight, sunny man called Joe, told us he'd never taken a fare to find 35m-year-old rock before. Where in Texas had the asteroid fallen? Not in Texas, Jenniskens said. It fell on the east coast, but this particular impact had been significant enough to send molten rock about a third of the way across North America. Joe and I thought about that for a while. "What bothers me," Jenniskens said, "is that it's the scary aspect of meteorites that is put forward." He meant by film-makers, by novelists, by journalists. "Because it makes people sit up and go, oh wow! But flying in a plane is scary. Getting in a car is scary."

The site was a stretch of chalky ground between two farms. Jenniskens had hunted for asteroid treasure here before, and been successful. He instructed me to scour the pale ground for small, dark pieces of rock that were translucent when held to the light. "Like beer-bottle glass," he said.

If a city-killing asteroid made land, or burst apart low enough in the sky, 'it would be similar to what you’d have after a nuclear explosion'. Photograph: Disaster Playground

Jenniskens' obsession with meteorites began in 1981, when he first went out stargazing in Holland and was lucky enough to see a lusty meteor shower: four distant space rocks streaked across the night sky in one hour's watching. At the same time, the chance in it bothered him. He wondered if asteroids couldn't be better mapped so that we could begin more accurately to predict their distant appearances in the sky, even their collisions with Earth. He made this his life's work, moving from the Netherlands to California to join Seti in 1995.

Some of his methods have been unconventional. Not long ago, he commissioned a private plane to get (very slightly) closer to a meteor shower. He said he'd once petitioned Hollywood studios to show more shooting stars in their films, an effort he felt had been a moderate success. Jenniskens is a love-your-meteorite campaigner of some 30 years' standing, and his enthusiasm for these things was infectious. Under the Texan sun, we searched for an hour, me bounding over every so often with a sample, mostly to be told it was beer-bottle glass. I lasted in the heat as long as I could before drifting back to Joe in his air-conditioned cab, where we fell into a discussion about film.

"For pure entertainment? Armageddon," Joe said. I told him I was more of a Deep Impact man. Did he ever think about the events of those movies coming to pass? "One or two times," Joe said. And? "If it happens, it happens. Nothing I can do." When he returned to the car, Jenniskens wouldn't accept this degree of fatalism. If it happens, it happens? That used to be the case, the astronomer corrected Joe, before detection advocates started chipping away at the divine chance in it all.

Jenniskens had found no asteroid debris today, but he'd brought some treasures with him in case. Opening his satchel, he showed me pieces of impact debris he'd recovered on an earlier visit to this part of Texas; also some tremor-smashed glass collected in Chelyabinsk and a fragment of the Chelyabinsk meteorite itself. It was magical to see it, this pebble from space, black as coal and studded with goosebumps that glittered faintly. It would have been up there for a billion-some years, orbiting between Mars and Jupiter before farcical chance brought it to Texas, via Russia, and into the satchel of a Dutchman.

There was an inevitability and even a beauty in asteroids coming to Earth, Jenniskens believed. He subscribed to the popular theory that, 4bn years ago, it was a bombardment of asteroids that first brought organic matter here, creating the conditions that made life possible. He told me the Earth most likely once orbited the sun on its own, until an asteroid the size of Mars banged in and tore us out a moon. The evidence was there that ours was a little planet that coped, moved on.

Take the largest meteorite ever to strike Europe, Jenniskens said: a mile-wide killer that plummeted into what is now France a few million years ago. It would have wiped out every living thing for 300 miles around, but on the site of that old catastrophe, there was now a beautiful little town with a chateau constructed from impactite, or rock that had been altered by the impact. As a building material, the rock is apparently excellent. "Asteroids can take life away," Jenniskens said. "And they can create it."

Rochechouart's main restaurant, honouring the area's ancient devastation, is called La Météorite. Diners can order Jupiter salad with a glass of Météor beer for about €15, and I ate this one afternoon in May, visiting the town on Jenniskens' recommendation. Rochechouart was indeed beautiful, its chateau especially, a 13th-century fortress on a hill with birds spiralling the turrets and donkeys grazing the fields beneath its walls. The town around it was just big enough to have a tourist office, a pair of competing bakeries and a saucy underwear shop. Until recently, I learned from an old photograph, there was a sign on the hillside road into Rochechouart that warned of falling boulders. But someone, perhaps an asteroid tourist, had taken it down.

I took a stroll, roughly tracing the width of the meteorite that had struck. The journey took me along a twisting road through a couple of villages and back; a long walk, because I couldn't help pausing to peer at the sky, wondering at the sight of it, a mile of rock bearing in. Lightly buzzed on Météor beer, I passed information boards that highlighted the rich geology of the area, the diversity of plant life rare for the region and almost certainly an accidental gift from the old meteorite. On the way back through town, I found a museum devoted to the strike, a poster by the door reading: "Imagine the worst kind of seismic upheaval!" Inside, I told the curator I'd been doing just that.

She let me know I wasn't alone, leading me to a display of drawings by young visitors. This collage of felt-tipped pages took up three walls and depicted, without exception, meteorite-brought hell. Six-year-old Alexandra had drawn the fiery destruction of her apartment block, and five-year-old Robin the last moments of a small town. Alyesee's drawing included a dinosaur with a speech bubble – "Attention! Meteorites!" – and Kylian had sketched a Nasa astronaut, hopelessly outmatched. One young artist, Adrienne, imagined a blatant civilisation-ender, her asteroid as big as the Earth. It was on a direct collision course, no chance it would miss.

Hot rocks: a brief history of asteroids and the Earth

The crater caused by a meteor strike in Peru in 2007 Photograph: AFP/Getty Images

4.5bn years ago The Earth gets its moon (according to the "big splash" hypothesis) when an asteroid the size of Mars breaks off a chunk of our planet.

2.4bn years ago An asteroid lands on what is now the Karelian district of Russia, leaving a 16km-wide crater, the oldest identified.

65m years ago The dinosaurs are done for, when a 10km-wide rock strikes what is now the Yucatán Peninsula in Mexico.

1790 Reports of a cottage crushed in the French town of Barbotan, killing a farmer and his cattle.

1868 Thousands of small stones shower down on Pultusk in Poland after a meteorite explosion in the sky; still bought and sold by collectors today, they are called Pultusk Peas.

1908 Hundreds of square miles of trees in Tunguska, Siberia, are flattened by a multi-kiloton explosion; some of the trees are even stripped of their bark.

1954 A fragment of meteorite bursts through the roof of Elizabeth Hodges' home in Alabama and hits her on the hip, briefly hospitalising her – the only confirmed case of a human struck by space rock.

1996 A big hit in rural Honduras leaves a 50m-wide hole.

2007 An explosion in Peru creates a 13m-wide hole and knocks a villager off his bike; after brief talk of fallen US satellites, and even missiles from neighbouring Bolivia, a meteorite is identified as the culprit.

2012 A strike in San Francisco, close to the site of the 19th-century Californian gold rush, prompts a new gold rush of sorts as meteorite hunters descend on the area looking for fragments.

2013 The Chelyabinsk event hospitalises more than 1,000.

2014 The appearance this month of a 12m-wide hole in Nicaragua was attributed to an asteroid strike. "Seems unlikely," said a Nasa spokesperson.